The present invention is directed to a lamp ballast circuit, and more particularly to a ballast circuit wherein the magnetizing inductance of the output transformer is adjusted in order to compensate for the reactive power induced by a ballast capacitor.
Fluorescent lighting is a very common type of illumination. Fluorescent lamps function when an electrical arc is established between two electrodes located at opposite ends of the lamp. The electrical arc is established by supplying a proper voltage and current to the lamp. The lamp is filled with an ionizable gas and a very small amount of vaporized mercury. When the arc is established, collisions occur between the electrons and the mercury atoms, causing the emission of ultraviolet energy. The fluorescent lamps have a phosphorous coating on their inner surface, which transforms the ultraviolet energy into diffused, visible light.
In order to establish the electrical arc, and thus turn on the lamp, a high voltage is typically required. However, once the lamp has been turned on, a lesser voltage is required to maintain the lamp""s operation. In order to start and operate a fluorescent lamp, a fluorescent lamp ballast is used. Among other functions (such as limiting the current flow through the lamp once it has already been started), a ballast is a device which provides the appropriate voltage to establish the arc through the lamps.
FIG. 1 shows a schematic diagram of a prior art ballast circuit 100 which employs a DC supply source 150. The DC supply source 150 is coupled to a pair of enhancement mode n-channel MOSFET transistors 102 and 104, which form a half-bridge structure. When alternately turned xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d, transistors 102 and 104 provide an AC voltage signal to the lamps. The drain terminal of transistor 102 and the source terminal of transistor 104 are connected at node 106. Node 106 is further coupled to current blocking capacitor 114 and resonant inductor 112, which are coupled in series. Current blocking capacitor 114 prevents DC current from flowing through the lamps.
Resonant inductor 112 is further coupled to a primary winding 110a of transformer 110. Resonant capacitor 116 is coupled in parallel across primary winding 110a. Secondary winding 110b of transformer 110 is coupled to a series combination of capacitors 122, 124, 126 and 128 and fluorescent lamps 132, 134, 136 and 138 coupled together in parallel. Capacitors 122, 124, 126 and 128 control the current flow through lamps 132, 134, 136 and 138, respectively.
One problem which is experienced by prior art lamp ballasts is that the voltage and current signals generated at secondary winding 110b of transformer 110 are out of phase with respect to each other. As a result, the primary winding voltage and current signals of transformer 110 are also out of phase with respect to each other. Thus, the power transferred through transformer 110 to drive the lamps is comprised of both a real power component (i.e.xe2x80x94the power provided to the lamps) and a reactive power component, which is described below.
Reactive power exists in a circuit due to an imbalance between peak magnetic energy storage and the peak electric energy storage in a circuit. For instance, the capacitor in the circuit stores its maximum energy when the voltage is maximum. The inductor stores its maximum energy when its current is maximum, which occurs when the voltage is zero because of the 90 degree phase shift. Since the ballasting capacitors 122, 124, 126 and 128 and the lamp load require energy at different times in the AC cycle, additional energy must periodically be supplied by the circuit in order to balance the load (i.e.xe2x80x94the lamps). This energy, transferred in and out of ballasting capacitors 122, 124, 126 and 128 during each cycle, is the reactive power.
Because of the additional reactive power that is not transferred to the load itself, prior art circuits are configured with a transformer that is large enough to handle the additional reactive power. In the case of the circuit described in FIG. 1, the reactive power is transferred by transformer 110. In order to insure that the reactive power is always sufficient to balance the load, transformer 110 must be sized significantly larger than if it was not required to transfer reactive power. The overdesign of this transformer undesirably adds to the cost and to the physical size of the lamp ballast circuit.
Therefore, there exists a need for a lamp ballast circuit that reduces the reactive power transferred through the output transformer.
The present invention is directed to a lamp ballast circuit with an adjusted magnetizing inductance of the lamp ballast circuit transformer. By optimizing the magnetizing inductance of the transformer, the voltage and current signals received at the primary side of the transformer are brought in phase relative to each other, and the reactive power transferred by the transformer is substantially reduced or eliminated. Since the transformer is not required to transfer reactive power, it may be sized significantly smaller, thereby decreasing the cost and the physical size of the lamp ballast circuit.
The lamp ballast circuit comprises a DC voltage and current supply source. A pair of transistors is coupled to the DC supply source and is configured, upon application of the DC signal, to provide an AC voltage and current signal, wherein each of the AC voltage and current signals has a corresponding phase. A transformer includes a primary and a secondary winding, wherein the primary winding of the transformer receives the AC voltage and current signals. At least one lamp is coupled to the secondary winding via a capacitor. The circuit has an optimal magnetizing inductance such that the AC voltage and current signals received at the primary side of the transformer are substantially in phase with each other, thereby substantially reducing or eliminating the reactive power transferred by the transformer. In a preferred embodiment, the magnetizing inductance, Lm, is given as:
Lm=[1+(xcfx89sC1R1)2]/n2xcfx89s2C1,
wherein xcfx89s is an operating frequency, C1 is the capacitance of the capacitor, R1 is the resistance of the at least one lamp, and n is a secondary-to-primary side turns ratio of the transformer.
The above description sets forth rather broadly the more important features of the present invention in order that the detailed description thereof that follows may be understood, and in order that the present contributions to the art may be better appreciated. Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.